This application relates to waveguides integrated with semiconductor devices.
Light propagates in straight line paths known as rays. Rays are refracted, reflected, and scattered at material interfaces. In a dielectric waveguide, a high index core is surrounded by a lower index cladding layer, rays are confined in the high index core region by total internal reflection at the core/cladding interface. The reflected rays interfere with each other to form electromagnetic field patterns within the guide. Modes of the guide refer to field patterns that propagate in the core region without dispersion, i.e. changing shape.
For electrical detection of light in a waveguide, the light needs to be absorbed in the detector material and the photogenerated charge must be collected. To improve detection efficiency, the absorbing material is typically placed in contact with the waveguide in a configuration known as evanescent coupling. The detector coupling efficiency can be improved by including an intermediate matching layer between the waveguide core and the absorbing detector material. This configuration still suffers from scattering loss from the detector material and inefficient coupling due to mode repulsion and guiding in the matching layer.
Performance limits of a photodetector device are given by the gain bandwidth product. A photodetector with high gain will have diminished frequency response or bandwidth, and a fast photodetector will have reduced gain or signal. The speed of a photodectector is limited by the transit time of the slowest carriers across the detector""s active region.